Active Noise Control (ANC) systems attenuate undesired noise using feedforward and feedback structures to adaptively remove undesired noise within a listening environment, such as within a vehicle cabin. ANC systems generally cancel or reduce unwanted noise by generating cancellation sound waves to destructively interfere with the unwanted audible noise. Destructive interference results when noise and “anti-noise,” which is largely identical in magnitude but opposite in phase to the noise, combine to reduce the sound pressure level (SPL) at a location. In a vehicle cabin listening environment, potential sources of undesired noise come from the engine, the interaction between the vehicle's tires and a road surface on which the vehicle is traveling, and/or sound radiated by the vibration of other parts of the vehicle. Therefore, unwanted noise varies with the speed, road conditions, and operating states of the vehicle.
A Road Noise Cancellation (RNC) system is a specific ANC system implemented on a vehicle in order to minimize undesirable road noise inside the vehicle cabin. RNC systems use vibration sensors to sense road induced vibrations generated from the tire and road interface that leads to unwanted audible road noise. This unwanted road noise inside the cabin is then cancelled, or reduced in level, by using speakers to generate sound waves that are ideally opposite in phase and identical in magnitude to the noise to be reduced at the typical location of one or more listeners' ears. Cancelling such road noise results in a more pleasurable ride for vehicle passengers, and it enables vehicle manufacturers to use lightweight materials, thereby decreasing energy consumption and reducing emissions.
RNC systems are typically Least Mean Square (LMS) adaptive feed-forward systems that continuously adapt W-filters based on both acceleration inputs from the vibration sensors located in various positions around a vehicle's suspension system, subframe and body, and on signals of microphones located in various positions inside the vehicle's cabin. RNC systems are susceptible to spurious noises from the sensors adding to the total noise within the passenger cabin. Because RNC systems are typically feed-forward systems, the vibration sensor outputs are equalized by the LMS W-filters and are then amplified and sent directly to the speakers, where they become airborne anti-noise signals. Accordingly, all signals output by the vibration sensors (typically accelerometers) are radiated into the passenger cabin, where they are heard by the vehicle occupants. This also means any spurious, impulsive noises sensed by the accelerometers are amplified and radiated into the passenger cabin where they become audible to the vehicle passengers. In vehicles with RNC, when a rock directly strikes an accelerometer, a high amplitude signal is output from the accelerometer's one or more directional output channels, which is then filtered by a W-filter, amplified, and radiated into the passenger cabin. This creates an additional loud impulsive noise, rather than creating a quieter interior noise level.